Sitting predicts risk of death independently from a person's level of exercise. based on the average person who sits 55% of their day, how many in that group would have died after 13 years of follow-up?

13% of that group would have died after years of follow up. 

A person who sits fifty percent of his day without exercise lives a sedentary lifestyle. People who have sedentary lifestyles are at higher risk for acquiring cardiovascular diseases.